They are, according to the "Nesting Delimiters" table in:
http://dlang.org/lex.html
When exactly are they seen as nesting delimiters?

When they're used in delimited strings. That's the whole point of that
section. The examples are
q"(foo(xxx))" // "foo(xxx)"
q"[foo{]" // "foo{"
but they could have included something like
q"<foo{>" // "foo{"
- Jonathan M Davis

They are, according to the "Nesting Delimiters" table in:
http://dlang.org/lex.html
When exactly are they seen as nesting delimiters?

When they're used in delimited strings. That's the whole point
of that
section. The examples are
q"(foo(xxx))" // "foo(xxx)"
q"[foo{]" // "foo{"
but they could have included something like
q"<foo{>" // "foo{"
- Jonathan M Davis